As is well known in the semiconductor art, large area photovoltaic cells that convert solar radiation into useable electrical energy can be fabricated by sandwiching certain semiconductor structures between two electrical contact layers. The front side contact layer is typically a fine line metal collection grid to collect the available current and direct it to one or more contact pads. The back side contact layer generally is composed of a metal such as aluminum. The voltage produced across the contacts of a single cell photovoltaic cell, however, is insufficient for most applications. In order to satisfy the application-related demands for voltage ranges from 12 to 15 volts, it is necessary to connect solar cells in series. The most common arrangement is to provide a linear string of cells in a common assembly, with interconnecting tabbing conductors joining adjacent cells front to back to provide the electrical summing of the individual cell voltages required to reach the desired voltage output level.
In the commercial processes commonly used for module assembly using cells with both front and back contacts, several steps are required. Tabs are soldered on the front contacts of the cells individually, and then the cells are electrically connected by sequentially soldering them into the circuit. Next, being careful not to strain the electrical connections, the fragile electrical circuit cell assembly is transferred to an encapsulation work station. Finally, the cell circuit is encapsulated in the module. This process often requires three or more work stations with low throughput, making the assembly process a very significant factor in the overall production cost.
Attempts have been made to simplify the assembly process, as for example in the Nakagawa, et al. U.S. Pat. No. 6,248,948 disclosure entitled Solar Cell Module and Method of Producing the Same. Here, among other features, the cell is produced with both contacts being front side accessible for applying the series connection tabbing. While offering potential for more automated assembly into modules, the disclosure does not extend to the machinery necessary to automate the assembly process.
Hirschberg, et al.'s U.S. Pat. No. 6,294,725 entitled Wireless Solar Cell Array Electrical Interconnection Scheme, offers conductive epoxy as a substitute for wire bonding, but again does not extend to the machinery necessary to automate the assembly process.
Semi-automated machinery, such as the SPI-Stringer 1000™ semi-automated production machine and the SPI-Assembler 5000™ is available from Spire Corporation. Other manufacturers including Ascor Inc., Anaconda, Seishin Trading Co. Ltd., Solon AG, SunWare GmbH & Co KG, NPC Inc., and Solarnova are known to service the industry with tabbing, stringing and assembly machines.